Adduct crystallization has been used for many years as a way to purify Bisphenol A. The goal of the crystallization is to produce large adduct crystals that can be easily separated from the mother liquor and washed without significant breakage. Phenol-BPA adduct crystals can be formed by way of an evaporative crystallization process wherein a volatile liquid aliphatic hydrocarbon, such as pentane or hexane, is used as a coolant, as described in U.S. Pat. No. 4,927,978. The crystallization process is implemented in a crystallizer formed from a cylindrical vessel having a draft tube concentrically disposed therein. An impeller circulates the crystallizer contents down through the interior of the draft tube and up through the annular space defined between the outside of the draft tube and the inner walls of the cylindrical vessel. A conical member may be placed below the draft tube to prevent solids from building up on the floor of the vessel and to facilitate the transition from downward to upward flow.
The aliphatic hydrocarbon coolant, as well as the main crystallizer feed and a recycle feed, are each introduced into the crystallizer through separate sets of nozzles mounted around the circumference of the vessel at different heights. The set of nozzles for the main crystallizer feed is the lowest of the three sets, being located near the bottom of the vessel. Each of the nozzles for the main crystallizer feed radially extends into the lower portion of the draft tube. The set of nozzles for the recycle feed are located above the nozzles for the main crystallizer feed, and extend into the annular space defined between the outside of the draft tube and the inner walls of the cylindrical vessel. The set of nozzles for the aliphatic hydrocarbon coolant is the highest of the three sets, being located near the middle of the vessel.
In operation, the main crystallizer feed is introduced into the bottom of the draft tube, while the recycle feed enters into the annular region in the bottom half of the crystallizer vessel. The aliphatic hydrocarbon coolant enters the crystallizer in the annular area at a height sufficient to suppress its immediate vaporization, typically near the mid-point of the vessel. The contents of the crystallizer vessel are circulated down the draft tube and up the annular area using the impeller located inside the draft tube. The aliphatic hydrocarbon coolant entrained in the vessel contents vaporizes as it approaches the liquid surface at the upper end of the vessel, creating a boiling zone which cools the surrounding liquid and precipitates phenol-BPA adduct crystals. The resulting product slurry is removed from the bottom or near the bottom of the vessel via one or more product drain openings or drain nozzles, while the vaporized aliphatic hydrocarbon coolant exits from the top of the vessel. The phenol-BPA adduct crystals are separated from the product slurry and washed, and part of the liquid component separated from the washed crystals is used for the recycle feed.